# Listbuilding via API Key

Endpoints for adding and removing subscribers using an API key–based authentication model.
These operations are optimized for secure listbuilding scenarios where full API authentication is not required.

 - [POST /subscriber/signin](https://developers.klicktipp.com/listbuilding-api/listbuilding-via-api-key/api.subscriber.signin.md): Creates or updates a contact and associates the tag linked to the API key. **Notice:** On success, the endpoint returns a system-generated redirect URL depending on the opt-in configuration in KlickTi
 - [POST /subscriber/signoff](https://developers.klicktipp.com/listbuilding-api/listbuilding-via-api-key/api.subscriber.signoff.md): Unsubscribes a contact via the API key and prevents further communication.
 - [POST /subscriber/signout](https://developers.klicktipp.com/listbuilding-api/listbuilding-via-api-key/api.subscriber.signout.md): Removes the tag of a contact associated with the API key.